In general, a fuel cell system is a power generation system of supplying air and hydrogen to a fuel cell and generating electrical energy with an electrochemical reaction of hydrogen and oxygen by the fuel cell. For example, the fuel cell system produces power from a fuel cell power plant, a public house, a factory, or the like, and is used for driving a driving source, such as an electric motor, a vehicle, a vessel, a train, an air plane, and the like. The fuel cell system includes a stack in which fuel cells are stacked, a hydrogen supplier for supplying hydrogen to an anode of the fuel cells, and an air supplier for supplying air to a cathode of the fuel cells.
In the case of a polymer fuel cell, appropriate moisture is necessary so as for an ion exchange membrane of a membrane electrode assembly (MEA) to smoothly perform a function, and to this end, the fuel cell system adopts a humidifier for humidifying reaction gas supplied to the fuel cell stack. The humidifier humidifies air supplied from the air supplier by using moisture in high-temperature and humid air discharged from the cathode of the fuel cells and supplies the humidified air to the cathode of the fuel cells. Further, the fuel cell system includes a hydrogen recirculator for mixing hydrogen discharged from the anode of the fuel cells and hydrogen supplied from the hydrogen supplier, and supplying the mixed hydrogen to the anode again.
A flooding phenomenon, in which a large amount of condensate water is left in the anode may be generated in the fuel cell stack. The flooding phenomenon may block hydrogen inflow to the anode by the condensate water to drop cell voltages of the fuel cells.
Accordingly, the fuel cell system includes a water trap and a drain valve as condensate water discharge devices for discharging the condensate water left within the anode of the fuel cells to the outside. The water trap may store condensate water discharged from the anode, and the drain valve may selectively discharge condensate water within the water trap according to a detection signal of a level sensor provided inside the water trap. Here, the water trap is installed in a drain line connecting a hydrogen exhaust line and an air exhaust line of the fuel cell stack, and supplies condensate water to the humidifier in a state where the drain line is opened through the drain valve. However, the condensate water of the water trap flows into the humidifier and moisture re-flows into the fuel cell stack in the flooding condition of the fuel cell, so that the cell voltages of the fuel cells by the flooding phenomenon of the fuel cell may be more seriously decreased.
During high-power driving of the fuel cell stack, high-temperature and dry air is discharged from the fuel cell stack, and there may incur a dry-out phenomenon in which the amount of moisture within the stack is insufficient due to excessive supply of air or high-temperature driving. The dry-out phenomenon may increase electric resistance of the cathode, thereby increasing deviation of a cell voltage. Particularly, movement momentum of air excessively supplied during the high-power driving of the fuel cell stack discharges even moisture necessary in the cathode, and decreases relative humidity of supplied air due to an increase in the amount of saturated water vapor under a high-temperature condition, thereby causing a dry-out phenomenon of the fuel cell stack.
The aforementioned humidity management of the fuel cell stack corresponding to the flooding condition and the dry-out condition of the fuel cell is variously evaluated through a regulative method, and the regulative method has an advantage in that the flooding condition and the dry-out condition of the fuel cell may be improved without adding a separate component. However, the regulative method may degrade fuel efficiency due to additional use of power and fuel according to an operation of a heater, or additional purging of hydrogen and supply of air under the flooding condition of the fuel cell, and performance of forced cooling, forced load, air pressure, and the like under the dry-out condition of the fuel cell.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.